In recent years, non-aqueous electrolyte secondary batteries that are charged and discharged as Li ions move between the negative electrode and the positive electrode are anticipated as large-sized energy storage devices for stationary type power generating systems such as photovoltaic power generation, from the viewpoint of environmental problems and the like.
Such non-aqueous electrolyte secondary batteries are required to have characteristics of withstanding the use for a long time period, or a low risk of ignition or explosion in the event of accident, at a level equal to or higher than that of those non-aqueous electrolyte batteries used in small-sized mobile telephones, notebook PCs and the like.
Non-aqueous electrolyte secondary batteries using polyanionic compounds as the positive electrode active material have high cycle stability and safety, and have therefore been practically applied as a stationary type power source. However, polyanionic compounds generally lack electron conductivity, and it is difficult to use the compounds by themselves. Therefore, positive electrode active materials produced by coating a polyanionic compound with a carbon material having high electron conductivity such as graphite are known. A positive electrode active material coated with a carbon material has excellent electron conductivity; however, the material lacks Li ion conductivity, and its rate characteristics are deteriorated when the material is excessively coated. In addition, since the carbon material used to coat the polyanionic compound does not contribute to the Li ion insertion and extraction, there is a problem that the charge-discharge capacity of the battery is deteriorated.